The present invention relates to a serial printing apparatus. The apparatus includes a sheet feed mechanism, which feeds a recording sheet at a predetermined pitch, and a printing head, which is mounted on a reciprocating carriage to form dots on the sheet.
A serial printing apparatus, such as an ink jet printing apparatus for color printing, has a carriage on which an ink jet printing head is mounted. The head has plural nozzle arrays, usually, four or more nozzle arrays, that eject differently colored ink droplets. The head moves in a main scanning direction and ejects the ink droplets on a recording sheet in response to printing data. When one scanned image is printed, a sheet feed mechanism feeds the recording sheet at a predetermined pitch. The apparatus alternately prints one scanned image and feeds the sheet.
Since the nozzle-pitch is extremely small, due to production improvements, the head prints at a resolution of 1440 dpi or higher. Moving the carriage continuously improves the accuracy of the printing position and the printing density in the main scanning direction. However, since the sheet feed mechanism intermittently drives in the sub-scanning direction, it is difficult to improve the positioning accuracy because of backlash.
The sheet feed mechanism has a sheet feed roller. The roller includes a driving shaft, which is connected with a driving motor through a transmitting means such as a set of gears, and a nonskid, elastic material made from rubber for covering the shaft. The backlash caused by the transmitting means and eccentricity of the roller decreases the accuracy of feeding sheets.
In order to solve these problems, a method for printing in which a sheet is continuously moved has been proposed. However, in this method the sub-scanning direction is inclined relative to the main scanning direction. Therefore, the sheet must be trimmed after printing. Also, the relative inclination of the scanning directions complicates the carriage moving mechanism.
Accordingly, it is an object of the present invention to provide a serial printing apparatus that improves printing accuracy in the sub-scanning direction without complicating the carriage moving mechanism and the sheet feed mechanism.
The present invention provides a printing apparatus that includes a printing head, a sheet feed mechanism and a displacement mechanism. The printing head reciprocates along an axis for performing printing passes. The sheet feed mechanism incrementally feeds a sheet in a direction transverse to the axis. The distance of each increment of sheet feeding is predetermined. The displacement mechanism shifts the printing head relative to the sheet in the sheet feeding direction or in a direction opposite to the sheet feeding direction to prepare for a printing pass. The distance by which the printing head is shifted to prepare for a printing pass is less than the distance of each increment of sheet movement. The displacement mechanism shifts the printing head between instances of sheet feeding.
The sheet feed mechanism feeds the sheet by the predetermined distance at which the mechanism can accurately feed. The displacement mechanism shifts the head relative to the sheet by the distance at which the feed mechanism cannot accurately feed. Accordingly, the apparatus finely prints with an accuracy.